This invention relates to a hologram recording and reproduction apparatus which records and reproduces a hologram in accordance with a shift multiplexing method, and more particularly to reduction in size and improvement in recording density of a hologram recording and reproduction apparatus of the type mentioned.
In the field of hologram memories which can achieve a large recording capacity by recording information three-dimensionally on a recording medium, various recording and reproduction methods such as angle multiplexing, shift multiplexing, wavelength multiplexing and phase modulation multiplexing have been heretofore proposed. Above all, it is preferable to adopt the shift multiplexing recording method for recording and reproduction of a hologram for which a rotational medium represented by a disk is used.
A conventional recording and reproduction apparatus which performs the shift multiplexing hologram recording has such a general configuration as shown in FIG. 5. Referring to FIG. 5, in order to record data on a hologram recording medium 12, a data page to be recorded is displayed on a spatial optical modulator (liquid crystal display apparatus of the light transmission type) 8 while a shutter 3 remains closed. Then, a spindle motor 24 is rotated to determine a recording place (recording area) of the hologram recording medium 12, and then the shutter 3 is opened.
Consequently, a coherent laser beam emitted from a laser light source 2 passes through the shutter 3 and enters a beam splitter 4, by which it is split into recording light 100 and reference light 200. The recording light 100 is introduced to the spatial optical modulator 8 by a mirror 6. When the recording light 100 passes through the spatial optical modulator 8 on which the data page is displayed, it is spatially optically modulated (amplitude modulated) by the spatial optical modulator 8. The modulated recording light is condensed on a recording area of the hologram recording medium 12 by a recording light lens (optical lens) 10.
Meanwhile, the reference light 200 is reflected to change its advancing direction by a mirror 14 and is then illuminated by a lens 18 so that it intersects in a fixed angle with the recording light 100 to generate interference fringes within the hologram recording medium 12. The data page (hologram) described above is recorded as a refractive index distribution according to a spatial distribution of the interference fringes.
After one hologram is recorded, the hologram recording medium 12 is moved by a fixed distance relative to the optical system, and then a next hologram is recorded. In the arrangement of FIG. 5, every time one hologram is recorded on the recording medium 12, the disk type hologram medium 12 is rotated by a fixed angle by the spindle motor 24. Then, after the hologram recording medium 12 makes one rotation, the optical system or the hologram recording medium 12 is moved in a radial direction of the hologram recording medium 12, and the recording in a circumferential direction of the medium is performed again. The sequence of operations described is repeated to record a large number of holograms over an overall area of the hologram recording medium 12.
In order to reproduce a hologram recorded in such a manner as described above, reference light (same as reproduction illumination light) of the same wave front is illuminated from the same position as that upon recording of the hologram on the hologram recording medium 12. Consequently, diffracted light corresponding to interference fringes recorded on a recording track of the hologram recording medium 12 is generated and is condensed by a lens 20 on and received by an image pickup element in a detector 22. Then, a resulting signal from the image pickup element is analyzed to restore the original image data (data page).
It has been proposed, for example, in Japanese Patent Laid-Open No. 2000-89648 that, where the hologram recording medium 12 is sufficiently thick when compared with the wavelength used for the recording, even if different holograms partially overlap with each other spatially, only a target hologram is reproduced if the shift amount between the holograms is greater than a fixed value. The fixed value of the shift amount relies upon the intersecting angle between the reference light 200 and the recording light 100, the f values of the individual lenses, the thickness of the hologram recording medium 12 and so forth. However, it is possible to realize, with regard to the along-track direction, a value of approximately several μm to several tens μm. It is to be noted that, where a spherical wave is used as the reference light, anisotropy appears and the shift selectivity in the cross-track direction becomes approximately 1 mm.
However, where cases of an optical disk recording and reproduction apparatus which are commercially established in the conventional field of optical memories are considered, a significant factor in apparatus miniaturization is miniaturization of an optical pickup. Meanwhile, in a hologram recording and reproduction apparatus of the shift multiplexing type, such elements as the lenses 18, 10 and 20 for reference light, recording light and reproduction light act similarly to the optical pickup of the optical disk recording and reproduction apparatus. Each of the lenses 18, 10 and 20 must be moved so as to follow up the information recording or reproduction position of the hologram recording medium 12. As the number of lenses to be moved is great, actuators for moving the lenses are obliged to be complicated and have increased sizes and servo systems are complicated, which decreases the degree of freedom in design. This makes it less easy to design a miniaturized apparatus. Thus, it is difficult to miniaturize a hologram recording and reproduction apparatus of the shift multiplexing type equivalently to existing optical disk recording and reproduction apparatus.
Further, the shift multiplexing is a method wherein the recording place of the hologram recording medium is successively moved in parallel little by little to perform multiplexed recording. However, since a spherical wave is used as the reference light, the recording density can be raised only in one direction due to the anisotropy. In particular, in the case of a disk type recording medium, while the shift selectivity of 10 μm can be taken in the along-track direction to achieve a high density, the shift selectivity in the cross-track direction is approximately 1 mm. Thus, there is a characteristic that the recording track pitch is great and the recording density cannot be raised.